For a charged particle irradiation equipment for irradiating an irradiation object with a charged particle beam (hereinafter referred to as “beam”), a charged particle beam irradiation equipment for irradiating the affected part of a patient with a beam to perform cancer treatment is known, and in Japanese Published Unexamined Patent Application No. 9-223600 (hereinafter referred to as the “first conventional art”), as an example, described is a charged particle beam irradiation equipment for applying the beam to each irradiation area in order after the affected part was divided into a plurality of the irradiation areas. The charged particle beam irradiation equipment described in the first conventional art controls an irradiation position of the beam with two scanning electromagnets for generating magnetic fields in two mutually orthogonal directions.
In the first conventional art, a configuration of a power supply for supplying power to the scanning electromagnets was not described in detail. However, one may consider a power supply for an electromagnet described in Japanese Published Unexamined Patent Application No. 8-88972 (hereinafter referred to as the “second conventional art”) is used for the scanning electromagnets. In this power supply, a filter for removing a pulsating component from its output is provided at the output side of the power supply in order to improve control accuracy of an exciting current flowing in the electromagnet.
As described above, since in the first conventional art, a plurality of irradiation areas are irradiated with the beam in order, the exciting current flowing in the scanning electromagnet increases or decreases in a stepwise manner, for example, as shown in FIG. 5(a). By the way, in the period of time when the current value is constant to the time, the irradiation position of the beam is kept fixed; in the period of time when the current value is changing to the time, the irradiation position is being changed over.
Referring to the period when the current value is changing to the time illustrated in FIG. 5(a), in other words, the period required for the irradiation position of the beam to be changed over, shorter is generally better. Because, as described in the first conventional art, if the irradiation of the beam is being stopped when the irradiation position of the beam is changed over, a period when the position of the beam irradiation is changed becomes dead time. If the dead time is longer, the treatment time will also be longer, causing a patient to be in burden and inconvenience. The time required for the beam irradiation position to be changed over, thus, must be as short as possible. On the contrary, if the beam irradiation is performed even when the irradiation position of the beam is being changed over, the irradiation dose applied in the period of time when the irradiation position of the beam is being changed over must be also considered to figure out the total dose at the affected part, and therefore it becomes difficult to make the irradiation dose at the affected part uniform. To circumvent this, the time necessary for the beam irradiation position to be changed over must be as short as possible so that the irradiation dose applied during the change of the irradiation position become so small as can be ignored. That is, the exciting current flowing in the scanning electromagnet must be changed in a very short time.
However, since the power supply stated in the second conventional art employs a filter for removing the pulsating component, a delay is generated in the output voltage and hence the exciting current flowing in the scanning electromagnet cannot be changed in a short time.
On the contrary, if a power supply without the use of a filter is employed, it may be possible that the exciting current can be changed in a short time, but the control accuracy of the exciting current flowing in the scanning electromagnet deteriorates under the influence of the pulsating component instead. If the control accuracy of the exciting current deteriorates, the irradiation position of the beam may be out of the right place, and therefore the beam cannot be applied uniformly to the affected part.